Add visitor to replace usages of postgres' FloatRangeFormField

[browniebroke]

From Django 2.2 release notes:

The FloatRangeField model and form fields in django.contrib.postgres are deprecated in favor of a new name, DecimalRangeField, to match the underlying numrange data type used in the database.

This is removed in Django 3.1

[codecov]

Codecov Report

Merging #198 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #198   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           21        22    +1     
  Lines          626       639   +13     
=========================================
+ Hits           626       639   +13     

Impacted Files	Coverage Δ
django_codemod/__init__.py	100.00% <100.00%> (ø)
django_codemod/visitors/__init__.py	100.00% <100.00%> (ø)
django_codemod/visitors/postgres_fields.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update e8d8182...37287d9. Read the comment docs.

[sourcery-ai]

Sourcery Code Quality Report (beta)

❌  Merging this PR will decrease code quality in the affected files by 0.01 out of 10.

Quality metrics	Before	After	Change
Complexity	0.05	0.05	0.00
Method Length	38.30	38.75	0.45 🔴
Quality	9.06	9.05	-0.01 🔴

Other metrics	Before	After	Change
Lines	311	321	10

Changed files	Quality Before	Quality After	Quality Change
django_codemod/visitors/init.py	8.32	8.21	-0.11 🔴
tests/test_cli.py	9.10	9.09	-0.01 🔴

Here are some functions in these files that still need a tune-up:

File	Function	Complexity	Length	Overall	Recommendation

Please see our documentation here for details on how these metrics are calculated.

We are actively working on this report - lots more documentation and extra metrics to come!

Let us know what you think of it by mentioning @sourcery-ai in a comment.